1. Field of the Invention
The invention concerns a method for correction of an image from a series of images acquired with an x-ray detector.
2. Description of the Prior Art
It is known to acquire a series of images using an x-ray detector formed by a number of detector elements so the acquired image is formed of image elements associated with the detector elements. A series of detector signals is generated in the detector element by x-ray radiation, the detector signals respectively exhibiting a detector-specific temporal decay curve. An image element to be corrected and acquired over a predetermined acquisition interval contains signal portions of the current and preceding detector signals acquired during this acquisition interval.
Regardless of developments in the field of medical technology, in particular the imaging modalities such as computed tomography and magnetic resonance tomography, conventional x-ray systems remain an important instrument for medical diagnosis and patient monitoring. Many presently-used imaging systems in the x-ray field are still equipped with analog detector technology. The field of digital imaging (in particular in C-arm x-ray systems), however, is of increasing importance. The imaging method and the devices used for translation thus are significantly relevant to the quality of the acquired images. The criterion of spatial resolution is known for evaluation of the quality of the imaging or of the imaging method, and quantum efficiency is known as a criterion for noise ratios or detector sensitivity. However, previously-used analog imaging offers barely any improvement potential in the region of quantum efficiency and the resolution capability. For example, due to other weak points of the analog technology a further reduction of the x-ray dose proves to be difficult. Furthermore, dependencies on the earth's magnetic field occur as well as non-linearity in the signal reproduction. The introduction of digital imaging is therefore increasingly being implemented, for example in the form of flat panel detectors. It is sought to replace the analog image chain (i.e. image intensifier, camera optic, camera and A/D converter) with an optimally simple, digitally-operating device. Advantages of digital technology are, for example, its dynamic range, i.e. in the resolution capability for fine tissue, further dose reduction of the x-ray radiation (which enables first 3D applications for soft tissue for C-arm systems), the mobility of the apparatus, and the linearity of the signal reproduction. Digital imaging methods include intrinsic methods, photoconductor methods and scintillator methods. Of these three significant methods, by today's knowledge the greatest improvement potential in 3D x-ray acquisition technology is seen in the scintillator methods.
With scintillators, detection of x-ray radiation ensues by secondary processes that are triggered by the x-ray radiation. If an x-ray quanta penetrates into a scintillator layer, among other things light emission is triggered by excitation and relaxation of luminophore centers in the scintillator layer. This light emission is detected by photodiodes by being generated and collected. Solid-state defects that decay again after a certain lifespan arise in the scintillator layer due to the x-ray radiation. Both luminophore emission and defect generation are directly proportional to the intensity of the incident x-ray radiation. The decay of the solid-state defects can lead to a re-excitation of the luminophore centers in the scintillator layer due to electron emission of the defect, which leads to a new signal detection by the photodiode. In the intervening time, however, new x-ray radiation can already have struck the detector. The detector signal generated from temporally-preceding radiated x-ray radiation thus overlaps with a detector signal generated from current x-ray radiation. The result is that a preceding image is visible in the current image in attenuated form. A disadvantage of scintillators thus is the high afterglow constant, or decay constant, which is dependent on the defect ratio in the scintillator layer. Similar effects as for the scintillator also occur for the photodetector. The decay constants for the photodiode are smaller, but the initial intensity is significantly higher than in the scintillator. Both proportions of the afterglow therefore play a relevant role given today's time intervals for image acquisitions. The decay curves for scintillators and photodetectors overlap into a common decay curve which determines the decay behavior of the acquired image.
A test with regard to shadow (ghost) images for a silicon planar detector is known from the technical article “A ghost story: Spatio-temporal response of an indirect-detection flat panel imager”, published in Med Phys. 26 (8) in August 1999. This test sought to remedy the occurring shadow images for a silicon planar detector by means of a rapid scanning method and a flood field method. Neither method delivered a satisfactory result. In the summary the author refers to the fact that the elimination of shadow images remains the subject of future work.